The present invention relates to an electrode employed in a solid polymer electrolyte type PEM (Proton Exchange Membrane) fuel cell and a direct type methanol fuel cell.
The PEM fuel cell is expected to be a compact and high output-density fuel cell. Followings are several methods for preparing this electrode of the PEM fuel cell.
(1) A method consisting of forming an electrode sheet by mixing electrocatalyst particles prepared in advance and polytetrafluoroethylene, and thermally bonding this sheet onto an ion exchange resin membrane under pressure (prior publications: U.S. Pat. Nos. 3,134,697, 3,297,484 and 3,432,355).
(2) A method of precipitating catalyst particles near the surface of an ion exchange resin membrane by means of chemical reduction (prior publications: Japanese patent publication gazettes Nos. 55-38934 and 58-47471).
The utilization ratio of the catalyst of the above methods are low, and a platinum catalyst amount of up to 20 mg/cm.sup.2 is required so that an inexpensive electrode for a fuel cell cannot be realized.
Followings are indispensable factors of effective catalyst working.
(i) Catalyst possesses sufficient electric contact with a membrane.
(ii) The catalyst exists in the ion exchange membrane as finely as possible, in other words it exists in high dispersion and the electric contact among the particles is maintained.
(iii) The membrane covering the surface of the catalyst possesses a sufficient thickness for keeping considerable ionic conductivity while it should not be too thick to prevent diffusion of a reaction gas to the catalyst.
However, the above conventional method (1) does not satisfy (i), and the method (2) does not satisfy (ii).
Recently, as improvement of the method (i), a certain degree of elevation of catalyst utilization has been intended by secondarily applying catalyst on a molded electrode sheet surface by means of sputtering (prior publication: E. A. Ticionelle et. al., J. Electrochem. Soc., 135 (1988) 2209).
Since, however, the catalyst exists in a considerable thin layer in this method, the catalyst gets deeply into the ion exchange membrane resulting in the insufficient supply of the reaction gas and in the remarkable lowering of the characteristics. The incorporation of the sputtering process lowers the preparation efficiency and lacks the mass productivity.
The required factors in the above (iii) are contradictory with each other, and the improvement of properties by optimizing conditions may have limitations.